Modes of Operation

Computerized Engine Control System:

The PGM-FI Control (ECU) Unit in this vehicle is a dedicated microcomputer which controls virtually all engine functions. Based on information received from the inputs, the ECU determines the current demands being placed on the engine. Once this is known, the ECU, based on the program contained in permanent memory, uses the ECU outputs to tailor engine performance to more efficiently meet those demands. A generalized list of the different control functions and the pertinent inputs and the outputs that control them are as follows:

INJECTOR TIMING AND DURATION
The ECU contains memories for the basic discharge durations at various engine speeds and manifold pressures. The basic discharge duration, after being read out from the memory, is further modified by signals sent from various sensors to obtain the final discharge duration. The most vital of these sensors are: Coolant Temperature (TW) sensor, Intake Air Temperature (TA) sensor, Oxygen sensor, and the Atmospheric Pressure (PA) sensor.

ELECTRONIC AIR CONTROL:
When the engine is cold, the A/C compressor is on, the transmission is in gear (A/T only) or the alternator is charging, the ECU controls current to the EACV to maintain correct idle speed.

IGNITION TIMING CONTROL:
The ECU contains memories for basic ignition timing at various engine speeds and manifold pressures. Ignition timing is also adjusted for coolant temperature.

OTHER CONTROL FUNCTIONS:

1. Starting Control
When the engine is started, the ECU provides a rich mixture.
2. Fuel Pump Control
a. When the ignition switch is initially turned on, the ECU supplies ground to the main relay which supplies current to the fuel pump for two seconds to pressurize the fuel system.
b. When the engine running, the ECU supplies ground to the main relay which supplies current to the fuel pump.
c. When the engine is not running and the ignition is on, the ECU cuts ground to the main relay which cuts current to the fuel pump.
3. Fuel Cut-off Control
a. During deceleration with the throttle valve closed, current to the injectors is cut off to improve fuel economy at speeds over 1500 rpm.
b. Fuel cut-off action also takes place when engine speed exceeds 7000 rpm, regardless of the position of the throttle valve to protect the engine from over-running.
4. A/C Compressor Clutch Relay
When the ECU receives a demand for cooling from the air conditioning system (compressor control unit), it delays the compressor from being energized, and enriches the fuel mixture to assure smooth transition to the A/C mode.
5. Purge Cut-off Solenoid Valve
When the coolant temperature is below 158°F (70°C), the ECU supplies a ground to the purge cut-off solenoid valve which cuts vacuum to the purge control valve.
6. Bypass Control Solenoid Valve (BPCSV)
When the engine rpm is below 5000 rpm the BPCSV is activated by a signal from the ECU, intake air flows through the long intake path, then high torque is delivered. At speeds higher than 5000 rpm, both solenoid valves are deactivated by the ECU, and intake air flows through the short intake path in order to reduce the resistance to the airflow.
7. EGR Control Solenoid Valve (EGR CSV)
When the EGR is required for control of oxides of nitrogen (NOx) emissions, the ECU supplies ground to the EGR CSV which supplies regulated vacuum to the EGR valve.

ECU BACK-UP FUNCTIONS:
1. Fail-Safe Function
When an abnormality occurs in a signal from a sensor, the ECU ignores that signal and assumes a pre-programmed value that allows the engine to continue to run.
2. Back-Up Function
When an abnormality occurs in the ECU itself, the injectors are controlled by a back-up circuit independent of the system in order to permit minimal driving.
3. Self-Diagnosis Function (Check Engine Warning Light, LED Indicator)
When an abnormality occurs in a signal from a sensor, the ECU lights the Check Engine warning light, stores the failure code in erasable memory and indicates the code with the LED on the ECU anytime the ignition is on. When the ignition is initially turned on, the ECU supplies ground for the Check Engine warning light for two seconds.